The role of the cell cycle in the human body
For normal functioning and the maintenance of homeostasis, the body constantly requires new cells. Every second, millions of new cellular elements appear in the human body, replacing damaged, aging, or dead ones. Altogether, the adult human body contains a colossal number of cells — approximately $5 \cdot 10^{13}$.
Clinical significance of this continuous process cannot be overstated. If cell division suddenly stops, the body loses the ability to renew its tissues. A striking example of this pathological condition is severe radiation sickness, in which the blockade of cell division leads to the inevitable death of a person in just a few days.
Duration of the cell life cycle
The time that elapses from a cell's birth to its division or death is not strictly fixed. It is variable and directly depends on two main groups of factors:
- Tissue type. Cells of different organs have completely different tasks and undergo varying degrees of wear and tear. For example, bone marrow cells live for 8–12 hours, skin epithelium is renewed in 20 days, and erythrocytes circulate in the bloodstream for up to 120 days.
- External factors. The rate of the cell cycle can change under the influence of ambient temperature, the presence of essential nutrients, and adequate oxygen supply ($O_2$). In the event of resource deficiency, the cycle can slow down or stop.
The mitotic cycle as part of the cell cycle
The most important component of the life cycle of a dividing cell is the mitotic cycle. This is a strictly ordered process that includes both the preparation of the mother cell for reproduction and the process of genetic material distribution itself.
The mitotic cycle is traditionally divided into two main stages:
- Interphase — a prolonged period of cell preparation for division. During this time, resources accumulate and genetic material is doubled. The average duration of interphase in humans is 16–24 hours.
- Mitosis — cell division itself, resulting in the formation of daughter cells. This is a faster process, taking an average of 1–2 hours.
Briefly: answers to topical questions
What periods is interphase of the mitotic cycle divided into?
Interphase of the mitotic cycle is divided into three periods:
- G₁ phase — the postmitotic (presynthetic) period.
- S phase — the synthetic period.
- G₂ phase — the premitotic (postsynthetic) period.
What happens during the S (synthetic) phase of interphase?
During the S (synthetic) phase of interphase, replication (doubling) of DNA and chromosomal proteins occurs, along with the formation of sister chromatids. Centrioles are duplicated near the nucleus. Meanwhile, the amount of DNA increases from $2n$ to $4n$. Initiation at each replication origin occurs strictly once, but centromeric regions of DNA are not replicated during this period.
What phases does the process of mitosis include?
The process of mitosis includes four sequential phases:
- Prophase — chromatin condensation, formation of the mitotic spindle.
- Metaphase — alignment of chromosomes at the equatorial plane of the cell.
- Anaphase — cleavage of centromeres and migration of chromatids to the poles.
- Telophase — reformation of the nuclear envelope, decondensation of chromosomes, and cytokinesis.
What is the G0 phase of the cell cycle?
The G₀ phase is a stage of reproductive quiescence. Differentiated (specialized) cells enter this state after their formation, exiting the active cell cycle, which is why they may lack the G₁ phase.
Which cells of the human body are in the G0 phase and do not divide?
Cells in the G₀ phase are terminally differentiated, non-dividing cells that lose the ability to transition from quiescence to the G₁ stage (proliferation). These include:
- Neurons.
- Cardiomyocytes.
- Rods and cones of the retina.
Class and exam questions
What is the difference between the cell cycle and the mitotic cycle?
The cell cycle is a broader concept encompassing the entire life of a cell from its appearance to death or division. The mitotic cycle is only that part of the cell cycle that includes preparation for division (interphase) and the division process itself (mitosis).
Why does a person die quickly from radiation sickness?
Radiation blocks the ability of cells to divide. Since millions of new cells must be formed every second to maintain homeostasis (e.g., in the bone marrow or epithelium), halting this process causes rapid tissue destruction and death of the organism within a few days.
What determines how long a cell will live?
This is primarily determined by the tissue type (e.g., skin cells live for 20 days, while erythrocytes live for 120 days). External conditions also play an important role: temperature, availability of nutrients, and oxygen.
What else to study on this topic
- Mechanisms of cell preparation for division in interphase
- Detailed analysis of the stages of mitosis
- The effect of hypoxia and nutrient deficiency on the speed of the cell cycle
- Cell death processes within the life cycle
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